It's unclear where or when the photo was taken, but Aniston was on Howard Stern's show last week and mentioned that she and her former costars were able to grab dinner together recently ... so it might be from that outing.
Jen captioned her pic, "And now we're Instagram FRIENDS too. HI INSTAGRAM."
The weird thing is her account was temporarily taken down after being live for less than an hour, but it's up and running again. A few notable stats ... she's already over 120k followers, and she's following her ex, Justin Theroux.
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It appears Aniston commented on LeBlanc's most recent IG post too ... which was a 1994 cast photo he shared last month in honor of the 25th anniversary of the "Friends" premiere.